In an ion trap mass spectrometer, the ion population collected by the ion trap during an analytical scan is typically regulated using a technique called automatic gain control (AGC). More specifically, before the analytical scan, a prescan is carried out by opening the gate of the ion trap for a predetermined time interval, and then determining the population of ions collected during that time interval. This ion population is typically referred to as the total ion current (TIC). Based on the TIC determined for the prescan time interval, an ion injection time is determined for use during the subsequent analytical scan. The ion injection time is determined with the goal of filling the ion trap to a point where it contains a desired number of ions, sometimes referred to as the AGC target value. In this regard, each ion trap has an AGC target value associated with it, representing approximately the maximum number of ions that the ion trap can hold without producing undesirable effects, such as where ions with a large mass-to-charge ratio (m/z) cause space charge effects for lower m/z ions.
It is known in the art that the gate of an ion trap must be open for a certain minimum period of time before the ion trap will begin to collect ions. This minimum period of time is typically referred to as the gate offset time. In pre-existing systems, the gate offset time was assigned a constant value, such as 1.5 μsec, for the entire m/z range of interest. This 1.5 μsec offset time was added to the injection time calculated from the prescan data, in order to determine the gate time during which the gate would be open for the analytical scan. The analytical scan was then carried out using this gate time. Where the analytical scan was a full scan across a wide range of m/z, the number of ions trapped for each m/z would vary with the length of the calculated injection time. Consequently, the data collected during the analytical scan needed to be normalized, and was therefore scaled by dividing the detected ion intensity for each m/z by the injection time calculated from the prescan data. Although this conventional scaling technique has been generally adequate for its intended purpose, it has not been satisfactory in all respects.